The present invention relates to the field of synchronous hierarchy communications networks, for example synchronous digital hierarchy (SDH) and SONET networks and to the synchronisation of network elements within such networks.
A synchronous hierarchy communications network comprises a number of interconnected nodes or network elements (NE), e.g. SDH equipment (SE), arranged to exchange data, synchronisation and control signalling according to a synchronous hierarchy, as set out for example in the synchronous digital hierarchy (SDH) or SONET standards. Typically a synchronisation signal will be passed from one NE to the next, along with the data signal, so creating a synchronisation path through communicating NEs via data ports. The synchronisation is carried by virtue of the synchronisation signal's aggregate bit rate rather than the data it contains. The signal also carries the synchronisation status message (SSM).
For brevity in the following the invention will be described with reference to SDH, it being understood that the invention is also applicable to SONET. A significant feature of SDH systems is the ability of networks to automatically recover from synchronisation failures. To support this feature each SE requires a pre-configured synchronisation source priority table and each synchronisation signal must carry an indication of the quality of the timing source from which it is derived. In SDH networks, this indication of quality is carried in the SSM. For any one STM-N output, the SE will autonomously select from the SSM values of the available sources the one with the highest quality. The selection of which source to use to synchronise data signals sent out from STM-N ports of an SE is also controlled by the pre-configured priority table of that equipment. For any one STM-N output, one or a number of synchronisation sources can each be assigned a priority and the SE can use the priority table to identify the source with the highest priority. In practice priority is only used to select a synchronisation source if there are more than one available with the same, highest quality, or if a valid SSM is not available.
At a network level it is important that the overall trail of successive synchronisation sources (which may be described as a “daisy chain” of SEs passing synchronisation information from one to the other along the chain) always refers back to a designated external source. This is illustrated in FIG. 1. It is also important that every effort is made to prevent a timing loop occurring. A timing loop occurs when a synchronisation signal transmitted by a SE is returned (i.e. looped-back) to the same SE which then selects that looped-back synchronisation signal as its source for transmitting thus “closing the loop” such that there is no independent external source of synchronisation. This is illustrated in FIG. 2.
To avoid this situation, international standards (e.g. pr ETS 300 417-6-1) define a method whereby, when a synchronisation source input to a SE via a particular port is selected for onward transmission by that SE, the SSM sent out by that particular port is set to “do not use” (DNU). A synchronisation signal with an associated SSM of DNU will not be used. This ensures that a synchronisation signal directed back towards the original source of synchronisation is not itself used for synchronisation.
However, the method of the above International Standard does not cope with the situation where two adjacent NEs are connected via more than one port. Whereas a SSM of DNU will be transmitted on the first port at which the synchronisation signal is received, the second port connected to the same source of synchronisation can still output a synchronisation signal with a valid (i.e. non-DNU) SSM.